erltrek_calc: minor tweaks to in_[sect|quad]* functions.
move [quad|sect]xy_index/1 to erltrek_calc.
erltrek_calc: add index_quadxy/1 function.
erltrek_scan: clean up condition_string/1.
erltrek_calc: add index_sectxy/1.
Merge pull request #15 from kaos/kaos

move [quad/sect]xy_index/1 to erltrek_calc
* Also clean up erltrek_scan:condition_string/1